Filled polyurethane foams of various kinds are already known. For example, U.S. Pat. No. 3,772,219 describes a soft polyurethane foam filled with powdered limestone which is an inexpensive starting material suitable for the production of foam chips which may be processed into composite chip material. A polyol filled with powdered limestone is used for the production of foams of this kind, being mixed and reacted with a polyisocyanate in the usual way. One of the disadvantages of the process, however, is that the viscosity of the polyol is increased by the inorganic additive, with the result that in conventional foam recipes, it is only possible to use polyols of very low viscosity. In addition, the burning properties of the resulting foams are adversely affected, because no polyol can be saved and the filler has a wicklike effect. Finally, the quantity of limestone which can be introduced into the system is heavily restricted, mainly due to viscosity problems.
According to Japanese Patent Application 48/49 842 (as laid open to inspection), wood substitutes (for example boards) are produced by a pressing technique from mixtures of an inorganic filler, (for example gypsum), and a mixture of polyurethane foam components. Although these products would appear to behave favorably in the presence of a flame, they cannot be produced by conventional foam-producing methods and, once again, contain as one of their constituents a polyether which has an adverse effect with regard to flame propagation and smoke gases. In addition, these products are obviously not foams in the accepted sense, but are more like weakly blown, highly filled plastics.
The same disadvantages apply to the floor-covering compositions described in German Offenlegungschrift No. 2,254,251, which are produced by mixing a filler, such as calcined porcelain earth, glass beads, sand or gravel, with liquid polyurethane-forming mixtures, and by then molding the resulting mixture. In addition, this process obviously utilizes coarsely divided inorganic materials rather than finely divided fillers.
According to French Pat. No. 2,147,839, filler-containing polyurethane foams are produced by impregnating preformed filler-free polyurethane foams with a suspension of a finely divided inorganic filler together with an organic binder. It is clear that a multistage process of this kind is not very attractive from the economic point of view.
It is also known (cf. German Offenlegungsschriften Nos. 2,319,706; 2,328,610 and 2,356,920) that reaction products containing terminal isocyanate groups, of hydrophilic, water-soluble, polyethylene glycol polyethers and polyisocyanates (NCO-prepolymers) can be foamed with a large excess of water. The water used for foaming may also contain inorganic fillers or sinterable ceramic powders. Mixing the highly hydrophilic NCO-prepolymers with the aqueous suspension results initially in the formation of a homogeneous aqueous solution of the isocyanate which contains the filler in suspension. Although the hydrophilic character of the organic components provides extremely good foaming conditions, and although considerable quantities of inorganic fillers may be used without causing viscosity problems, the foams obtained are, of course, also hydrophilic. They are able to absorb considerable quantities of water and, in doing so, swell to a considerable extent. At the same time, the products tend to soften. This range of properties provides numerous potential applications, for example the production of moisture-absorbing materials, materials for the cultivation of plants, hygienic articles, hydrophilic finishes in woven and non-woven textile materials, and the like. On the other hand, materials of this kind cannot be used for applications requiring low water absorption, dimensional stability and long-term stability under the effect of moisture.
It is also known (cf. for example German Offenlegungsschrift No. 2,113,042) that cement compositions or mortars can be produced by mixing an hydraulic cement, a silica filler especially sand, water and a polyisocyanate and molding the pasty mixture thus obtained. Polymer concretes having a high resistance to chemicals are obtained in this way, being used in particular for the production of floor coverings. The products are compact or weakly porous. Foams cannot be produced by this process. The processing technology corresponds to that of concrete.
Finally, it is known that inorganic-organic foams can be produced from polyisocyanates and aqueous solutions of alkali silicates. Hydraulic or inert powder-form inorganic fillers may also be used. In this process, the aqueous solution of the alkali silicate is an essential constituent, especially so far as stabilizing the foam is concerned. It is possible by this process to produce excellent hard foams which are suitable, for example, for the building sector. One disadvantage of this process, however, is that the compatibility of aqueous alkali silicate solutions with a number of fillers is inadequate. The fillers either have to be mixed with the polyisocyanate or have to be added in dry form as a third component. This procedure seriously restricts the production of products of this type in conventional foaming machines. In addition, the mixture of polyisocyanate and fillers causes storage and stability problems.
It has already been proposed (Belgian Pat. No. 822,697) to produce foams from ionic polyisocyanates, water and inorganic fillers. Foams of this kind are substantially free from water-soluble salts and behave very favorably in the presence of a flame. Calcium hydroxide suspended in water, for example, has been used to fill foams of this kind. Foams of this type are readily produced by mixing the ionic isocyanate with water and the inorganic filler. Blowing agents may also be added. The resulting products generally have densities greater than 200 kg/m.sup.3 and show very good mechanical properties, and more especially, high compressive strength. The limits to this process for the production of highly filled polyurea ionomers are imposed both by the nature of the polyisocyanates used and also by the property spectrum of the foams obtained. For example, it is not possible to produce satisfactory highly filled lightweight foams of the kind required for insulating purposes or as shock-absorbing packing foams. Densities of less than 200 kg/m.sup.3 can only be obtained with difficulty and also result in an unfavorable coarse-pored cell structure, so that the foams obtained show an inadequate heat-insulating capacity.
In addition, difficulties arise when standard commercial-grade unmodified and/or hydrophobic polyisocyanates are used instead of the ionic polyisocyanates. Mixing difficulties arise, and, the emulsions of water, filler and polyisocyanate initially formed tend to disintegrate. In addition, it is difficult to obtain adequate activation which is a particularly important factor with regard to the physical instability of the primary emulsion. Since both hard insulating, lightweight foams and also shock-absorbing, packing foams are required to show a high degree of hydrophobicity and dimensional stability under the action of water, it is not possible to use hydrophilic polyisocyanates of the type described, for example, in German Offenlegungsschrift No. 2,319,706.
For the reasons explained above, it has not yet been possible to produce lightweight polyurea foams (i.e. densities of from 10 to 200 kg/m.sup.3) from aqueous filler suspensions and hydrophobic polyisocyanates on a commercial scale, although foams of this type are of considerable interest from the economic point of view, and in addition, show extremely favorable burning properties.
Hitherto, it has only been possible to produce highly filled polyurea foams in combination with relatively large quantities of waterglass. The waterglass acts as reactant and, because of its high reactivity and its gellability, enables highly filled polyurea/silicate foams to be produced. On the one hand, however, waterglass is incompatible with a number of fillers, as already mentioned, so that it is not possible to prepare premixes and the filler has to be separately added as a third component. On the other hand, the use of waterglass may be undesirable, for example, because the foam, as a result, contains relatively large quantities of salts and because the dimensional stability of the foams under the effect of moisture is unsatisfactory. Accordingly, it would be desirable to be able to produce highly filled inorganic-organic polyurea foams without any need to use waterglass as reaction component. However, since waterglass behaves almost ideally in regard to its handling properties, stability, pumpability and miscibility and also in regard to its emulsifiability, and shows outstanding foamability with polyisocyanates, there is a commercial need to be able to use inexpensive inorganic fillers in a corresponding manner as a reactive component for the production of hydrophobic polyisocyanate-based lightweight foams. The present invention provides a solution to this problem.